The present invention relates to a packet-switched communications system which is capable of high speed, high throughput switching operations to handle a variety of traffic patterns by efficient utilization of system resources.
According to a prior art packet switched communications system, flow control with a higher-level data link control and error recovery retransmission control are effected on a link-by-link throughout the network. These techniques are needed because the transmission speed of the prior art packet switching system is low, typically in the range between several Kbps and several tens of Kbps and the bit error rate is relatively high. Since these techniques involve a large number of complicated procedures, they require a lengthy period execution time. In addition, the execution of such procedures must be repeated at locations where packets are retransmitted over the network, resulting in packets arriving at delayed times.
The introduction of optical fibers as a high speed high quality transmission medium allows reduction of packet transmission delay. However, the amount of delay involved in the execution of link-by-link flow control and retransmission is becoming a dominant factor of the total delay time. On the other hand, the frequency at which the retransmission process must be effected has decreased significantly with the reduction of transmission bit error rate and the control overhead for link-by-link retransmission increases significantly. As a result, a high speed, high throughput packet switched communications system cannot be implemented with the introduction of high speed, high quality transmission media without improvement of the switching speed of the packet switching system,
More specifically, FIG. 1 shows a packet transfer sequence effected between end terminals according to a prior art packet switched communications system. In FIG. 1, character L represents the execution of flow control and retransmission control, on a link-by-link basis, L(S) indicates a transmission execution and L(R) indicates a reception execution. Character N represents a packet switching operation by means of which a route is determined for each packet. D#n represents the packet and ACK(L) and ACK#n represent link-by-link acknowledgement and end-to-end acknowledgement, respectively. RGJ(L)#n represents a request for retransmission between links and ERR indicates that a packet has been affected by an error during transmission. The number of frames which can be continuously transmitted between switching offices, or "nodes" and the number of frames which can be continuously transmitted between end users are both assumed to be "8". Since no acknowledgment ACK is returned to packets D#0 to D#14 generated by a terminal PT(A) and eight packets can be continuously retransmitted, the first eight packets D#0 through D#7 are allowed entry to the network. Acknowledgement ACK(L) is returned at each link and acknowledgement ACK#n is returned to the source terminal to indicate that the destination terminal has correctly received packets D#0 through D#n. Assume that an error has occurred in packet D#4 during transmission between nodes PS(A) and PS(B) and node PS(B) returns a request for retransmission REJ(L)#4 to node PS(A) and the latter retransmits packets D#4 through D#7 to node PS(B). When acknowledgment ACK#4 is returned to source terminal PT(A), the remaining four data packets are allowed entry to the network.
As can be seen from FIG. 1, the amount of processing time at each node is substantial particularly when speech signal is transmitted. Because of the redundancy of information in the speech signal, no retransmission is required even if packets are noise affected at low frequency. Although buffer overflow in an interface node can be avoided, it is impossible to directly restrict the traffic at the entry point of the network when overflow occurs in an intermediate node. As a result, the traffic congestion in an intermediate node is likely to migrate to neighboring nodes.